1. Field of the Invention
This invention relates to an auto-focusing method and particularly to a method to be applied to an auto-focus system of a photographic camera.
2. Description of the Prior Art
An auto-focusing method for carrying out an auto-focusing operation by leading the light reflected from a subject to be photographed to a pair of solid-state image sensors through a lens to be brought into in-focused position and displacing the lens in the direction of optical axis in accordance with a correlation between the outputs from the solid-state image sensors is well known in the art. A typical example includes Honeywell's TLC system.
In this type of the prior art auto-focusing method, it is assumed that the output from each of the pair of solid-state image sensors provide the same functional format so that it is highly susceptible to noises. Furthermore, if output characteristics of a plurality of photoelectric elements of each of the solid-state sensors are scattered, the outputs, which should inherently be of the same functional format, are distorted by such a scatter and differ one from another. For this reason, it is thus required to use a solid-state image sensor having no scatter in the output characteristics of its photoelectric elements for each of the paired solid-state image sensors. However, since the yield in manufacturing such a solid-state image sensor is low, it is difficult to lower the cost of such a solid-state image sensor, thereby pushing up the cost of auto-focusing system.
Moreover, in this type of prior art auto-focusing method, the correlation between the outputs from the paired solid-state image sensors is evaluated by the following evaluation function ##EQU1## where A(n) and B(n) are outputs from the respective solid-state image sensors for n=1-N with N indicating the number of photoelectric elements in each of the solid-state image sensors and .alpha., .beta. and k are set values. As is clear from the format of the evaluation function, it is affected by the contrast of a subject to be photographed. Thus, if the contrast is low, there are produced such disadvantages as deterioration in in-focusing accuracy and substantial time delay in realization of in-focused condition.